1. Field of the Invention
The present invention relates to a dual antiparallel (AP) pinned spin valve sensor biased by opposite ferromagnetic coupling fields and opposite demagnetizing fields and, more particularly, to a dual AP pinned spin valve sensor wherein ferromagnetic coupling fields exerted on a free layer by first and second AP pinned layer structures are antiparallel and net demagnetizing fields exerted on the free layer by the first and second AP pinned layer structures are antiparallel.
2. Description of the Related Art
The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a spin valve sensor for sensing the magnetic signal fields from the rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90xc2x0 to an air bearing surface (ABS) wherein the ABS is an exposed surface ofthe sensor that faces the rotating disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative magnetic signal fields from the rotating magnetic disk. The quiescent position of the magnetic moment of the free layer, which is preferably parallel to the ABS, is when the sense current is conducted through the sensor without magnetic field signals from the rotating magnetic disk. If the quiescent position of the magnetic moment is not parallel to the ABS the positive and negative responses of the free layer will not be equal which results in read signal asymmetry, which is discussed in more detail hereinbelow.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layers are minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with the pinned and free layers. When the magnetic moments of the pinned and free layers are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. An increase in scattering of conduction electrons increases the resistance of the spin valve sensor and a decrease in scattering of the conduction electrons decreases the resistance of the spin valve sensor. Changes in resistance of the spin valve sensor is a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layers. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals from the rotating magnetic disk.
The sensitivity of the spin valve sensor is quantified as magneto-resistance or magnetoresistive coefficient dr/R where dr is the change in resistance of the spin valve sensor from minimum resistance (magnetic moments of free and pinned layers parallel) to maximum resistance (magnetic moments of the free and pinned layers antiparallel) and R is the resistance of the spin valve sensor at minimum resistance. Because of the high magnetoresistance of a spin valve sensor it is sometimes referred to as a giant magnetoresistive (GMR) sensor.
The transfer curve for a spin valve sensor is defined by the aforementioned cos xcex8 where xcex8 is the angle between the directions of the magnetic moments of the free and pinned layers. In a spin valve sensor subjected to positive and negative magnetic signal fields from a moving magnetic disk, which are typically chosen to be equal in magnitude, it is desirable that positive and negative changes in the resistance of the spin valve read head above and below a bias point on the transfer curve of the sensor be equal so that the positive and negative readback signals are equal. When the direction of the magnetic moment of the free layer is substantially parallel to the ABS and the direction of the magnetic moment of the pinned layer is perpendicular to the ABS in a quiescent state (no signal from the magnetic disk) the positive and negative readback signals should be equal when sensing positive and negative fields that are equal from the magnetic disk. Accordingly, the bias point should be located midway between the top and bottom ofthe transfer curve. When the bias point is located below the midway point the spin valve sensor is negatively biased and has positive asymmetry and when the bias point is above the midway point the spin valve sensor is positively biased and has negative asymmetry. The designer strives to improve asymmetry of the readback signals as much as practical with the goal being symmetry. When the readback signals are asymmetrical, signal output and dynamic range ofthe sensor are reduced. The location of the bias point on the transfer curve is influenced by three major forces on the free layer, namely a demagnetization field HD from the pinned layer, a ferromagnetic coupling field HF between the pinned layer and the free layer, and sense current fields HI from all conductive layers of the spin valve except the free layer.
A dual spin valve sensor may be employed for increasing the magnetoresistive coefficient dr/R of a read head. In a dual spin valve sensor first and second pinned layer structures are employed with a first spacer layer between the first pinned layer structure and the free layer and a second spacer layer located between the second pinned structure and the free layer. With this arrangement the spin valve effect is additive on each side of the free layer to increase the magnetoresistive coefficient dr/R of the read head. In order to reduce demagnetizing fields HD from the first and second pinned layers on the free layer, each of the pinned layers may be an antiparallel (AP) pinned layer structure. The first AP pinned layer structure has an antiparallel coupling (APC) layer which is located between ferromagnetic first and second AP pinned layers (AP1) and (AP2) and the second AP pinned layer structure has an antiparallel coupling layer between third and fourth AP pinned layers (AP3) and (AP4). The AP pinned layers of each AP pinned layer structure have magnetic moments which are antiparallel with respect to one another because of a strong antiferromagnetic coupling therebetween. The AP pinned layer structure is fully described in commonly assigned U.S. Pat. No. 5,465,185 which is incorporated by reference herein. Because of the partial flux closure between the AP pinned layers of each first and second AP pinned layer structures, each AP pinned layer structure exerts a small demagnetizing field on the free layer. These demagnetizing fields, however, are typically additive since the magnetic moments of the AP pinned layers immediately adjacent the free layer structure must be in-phase (parallel with respect to one another) in order for the spin valve effect to be additive. Further, the magnetic moments of the AP pinned layers spaced from the free layer by the spacer layers exert ferromagnetic coupling fields HF on the free layer which are also typically additive and parallel to the demagnetizing fields HD. Accordingly, a net demagnetizing field HD, which is the total of the demagnetizing fields from the AP pinned layer structures, and a net ferromagnetic coupling field HFC, which is the total of the ferromagnetic coupling fields, act on the free layer. The net demagnetizing field and the net ferromagnetic coupling field are typically additive to urge the magnetic moment of the free layer structure from its zero bias position parallel to the ABS. There is a strong-felt need to counterbalance these magnetic fields on the free layer so as to obtain proper biasing.
The present invention provides a dual AP pinned spin valve sensor with a structure which causes the ferromagnetic coupling fields from the first and second AP pinned layer structures to oppose or counterbalance each other and demagnetizing fields from the first and second AP pinned layer structures which oppose or counterbalance each other. In order to cause the ferromagnetic coupling fields to oppose one another, the first and second spacer layers are appropriately sized in their thickness so that one of the ferromagnetic coupling fields has a negative polarity and the other ferromagnetic coupling field has a positive polarity. In a preferred embodiment this is accomplished by employing platinum manganese (PtMn) for the first and second pinning layers with first, second and third seed layers for the first pinning layer wherein the first seed layer is composed of aluminum oxide (Al2O3), the second seed layer is composed of nickel manganese oxide (NiMnO) and the third seed layer is composed of tantalum (Ta). The demagnetizing fields from the first and second AP pinned layer structures oppose one another by appropriately sizing the thicknesses of the first and second AP pinned layers of the first AP pinned layer structure and the third and fourth AP pinned layers of the second AP pinned layer structure. In a first embodiment of the invention the second AP pinned layer is thicker than the first AP pinned layer and the third AP pinned layer is thicker than the fourth AP pinned layer. In another embodiment the first AP pinned layer may be thicker than the second AP pinned layer and the fourth AP pinned layer may be thicker than the third AP pinned layer. In both embodiments a pulse may be conducted through the sense current circuit for setting each of the first and second pinning layers. Further, sense current fields from the conductive layers of the spin valve sensor will support the pinning direction of the magnetic spins of the first and second pinning layers when the sense current is appropriately directed so as to enhance thermal stability of the read head. In a preferred embodiment the magnetic thicknesses ofthe first and fourth AP pinned layers are equal and the magnetic thicknesses of the second and third AP pinned layers are equal. With this arrangement the net demagnetizing field on the free layer is zero.
An object of the present invention is to provide a dual AP pinned spin valve sensor wherein ferromagnetic coupling fields from first and second AP pinned layer structures oppose one another and demagnetizing fields from the first and second AP pinned layer structures oppose one another.
A further object is to provide the aforementioned dual AP pinned spin valve sensor wherein the net ferromagnetic coupling field on the free layer is zero and the net demagnetizing field on the free layer is zero.